1. Field of the Invention
The present invention relates to a semiconductor device and method of fabricating it, and, in more detail, relates to a semiconductor device and method of fabrication provided with a capacitor of a simple stacked structure in which a ferroelectric film or insulating film of high relative permittivity is employed as the capacitor insulating film.
2. Description of the Related Art
In recent years, advances have been made in achieving higher integration and higher densities of semiconductor devices such as for example DRAMs (Dynamic Random Access Memories). Increasing the density and increasing the integration of a semiconductor device tends to decrease capacitance, lowering the reliability of the semiconductor device.
An effective method of increasing the capacitance of a capacitor of small area is to employ a ferroelectric film or insulator film of high relative permittivity as the capacitor insulating film. Ta.sub.2 O.sub.5, SrTiO.sub.3, (Ba.sub.(x) Sr.sub.(1-x)) TiO.sub.3 etc. are considered promising as insulator films of high relative permittivity used for capacitor insulating films. These insulating films have no hysteresis characteristic and have extremely high relative permittivity. The necessary capacitance can thereby be ensured, without making the construction of the capacitor device complicated, by employing these as the capacitor insulating film.
A known example of the construction of a capacitor in which a high relative permittivity insulating film is employed as the capacitor insulating film is shown in FIG. 1 of Y. Ohno et al, "A Memory Cell Capacitor With Ba.sub.(x) Sr.sub.(1-x) TiO.sub.3 (BST) Film For Advanced DRAMs" 1994 Symposium on VLSI Technology Digest of Technical Papers, 11.1, p.149.
A method of fabricating a capacitor as disclosed in this article will now be described with reference to FIG. 9.
First of all, a contact plug 902 is formed in a prescribed position of inter-layer insulating film 901, using the ordinary photolithographic technique and deposition techniques. This contact plug 902 is employed as a contact for the lower electrode (corresponding to the storage node).
Next, a laminated film 903 is formed (see FIG. 9(A)) on the surface of inter-layer insulating film 901 so as to contact this contact plug 902. This laminated film 903 is processed in a subsequent step to form the lower electrode. Laminated film 903 is constituted of an adhesive layer 904 to prevent peeling of the lower electrode from the inter-layer insulating film 901, a barrier layer 905 for preventing diffusion of carrier within the lower electrode, and an electrode layer 906. For example, a titanium (Ti) film can be employed for adhesive layer 904, a titanium nitride (TiN) film can be employed for the barrier layer, and a ruthenium (Ru) film can be employed for the lower electrode layer 906.
Next, lower electrode 907 is formed by patterning of laminated film 903 by RIE (Reactive Ion Etching) or a similar technique. After this, a side wall 908 (see FIG. 9(B)) is formed on the side face of lower electrode 907 by etching using RIE etc. after deposition of an insulating material on the entire surface of inter-layer insulating film 901.
Next, capacitor insulating film 909 is formed by depositing a Ta.sub.2 O.sub.5, SrTiO.sub.3 or (Ba.sub.(x) Sr.sub.(1-x))TiO.sub.3 on the surface of inter-layer insulating film 901, lower electrode 907 and side wall 908. The capacitor is completed (see FIG. 9(C)) by forming an upper electrode 910 by depositing for example Ru on the entire surface of insulating film 909.
FIG. 9(D) is a view showing part of the capacitor of FIG. 9(C) to a larger scale. In this capacitor, the step coverage of insulating film 909 is improved by forming side walls 908 on the side faces of lower electrode 907, thereby preventing short circuiting of lower electrode 907 and upper electrode 910. Also, oxidation of adhesive layer 904 by annealing performed in a subsequent step is prevented by the provision of these side walls 908.
However, this gives rise to the drawback that, if a side wall 908 is formed on the side face of lower electrode 903, the capacitance of the capacitor is lowered, since the contact area of electrode 903 and insulating film 909 becomes small.
If a side wall 908 is provided, only the upper surface of lower electrode 907 acts as a storage node; the side face does not act as a storage node (see FIG. 9(D)). That is, since the area of the storage node is decreased to the extent of the area of the side face, the capacitance is lowered to that extent. Consequently, even by employing a ferroelectric film or high relative permittivity insulating film as capacitor insulating film 909, it was still not possible to raise the capacitance sufficiently.